1. Field of Invention
The present invention relates to a method of controlling the scanning resolution of a scanner. More particularly, the present invention relates to a method of increasing the scanning resolution of a scanner through control of its driving system.
2. Description of Related Art
Due to the many recent advances in computer multimedia technologies, image-processing techniques have gone forward a giant step, leading to some innovative designs for computer peripheral products such as a scanner. In a few years' time, the scanner has developed from a palm-top device capable of scanning only black and white pixels to a full-color high-resolution scanning system. In general, the resolution of a scanner along the horizontal axis depends on the density of its charge-coupled device (CCD) while the resolution of the scanner along the vertical axis depends on the level of precision of its driving system.
To increase resolution along the vertical axis, the following methods are conventionally adopted:
(a) Driving System Modification:
FIG. 1 is a timing diagram showing shift gate clock (SH) pulse and motor pulse (MTP) provided by the driving system of a conventional scanner. As shown in FIG. 1, one SH pulse period is represented by T1, which is the time needed to expose a scanner's CCD once. Typically, a period T1 is about 10 ms. The motor pulses are signals for controlling the forward rotation of a stepper motor. In general, the smaller the stepping angle of the stepper motor, the higher the resolution of the scanner is. Hence, by using a stepper motor having a small stepping angle, a high scanning resolution can be obtained. However, the cost of making a stepper motor with a small stepping angle is relatively high.
In addition, when the scanner is performing low resolution scanning operations, pulsing rate or pulse per second (PPS) of the stepper motor increases, leading to a low torque. Low torque increases the probability of stepping loss, thereby resulting in distortion of the scanned image.
(b) Pixel Compensation:
This method relies on displaying an image formed on the CCD in a single exposure twice on a screen in each exposure cycle. Hence, resolution is doubled. However, the image thus obtained is no longer the real optical resolution.
(c) Micro-stepping Motion Control:
This method relies on the addition of an external driving circuit so that the original stepper motor is changed to a stepper motor capable of stepping forward in ½, ¼ or ⅛ of a step each time. Through the reduction of each forward step, resolution along the vertical axis is increased. However, the minimum micro-step that can be driven is ultimately limited by the external circuit. Moreover, when an ordinary step is subdivided into too many micro-steps, control becomes very difficult. The outcome is that distortion of the scanned image is unavoidable.